A new study at Washington University is expanding the film’s premise to include eating fast food for more than four months so that participants in the study will gain weight.

Volunteers must already be obese and be willing to add items like a Big Mac and fries or a Whopper with cheese, to their daily diet. In the study funded by the National Institutes of Health, researchers want to learn why some obese people have obesity-related sicknesses and others don’t.

Participants will be required to eat fast food until they increase their weight by 5 percent.

“We’re using fast food because we know the calorie content and the calories are easier to keep under control,” said chief researcher Dr. Samuel Klein. “We know how much fat, protein, carbohydrates” are in those foods.

The volunteers have five restaurant choices — McDonald’s, Burger King, Kentucky Fried Chicken, Taco Bell and Pizza Hut — where they will add 1,000 excess calories a day to their normal diet.

Researchers want to know why about a third of people who are considered obese never get high blood pressure, diabetes, high cholesterol, too much fat in the blood or other conditions associated with obesity.

“If we could find out why some people are resistant to the adverse metabolic effects of obesity, we might be able to develop better therapies and medications to break the link between obesity and metabolic diseases, such as diabetes and heart disease,” Klein said.

In medical terms, obesity is defined by a body mass index — a ratio of height and weight — of 30 or higher. On average, a BMI ranging from 18.5 to 25 is considered to be healthy.

Nationwide, more than one-fourth of adults were considered obese in 2009, according to the U.S. Centers for Disease Control and Prevention. An additional 36 percent were overweight. Obesity-related medical costs rose to about $92.6 billion in 2002, the agency reported. About half was paid by Medicaid and Medicare.

“Obesity is a very expensive disease,” Klein said. “It’s driving many of the diseases that are burdening our health system.”

Klein is head of the Center for Human Nutrition and medical director of the Weight Management Program at the university.

The study needs 30 test subjects — 15 who don’t have metabolic problems and 15 who do. A larger study could follow if findings warrant it.

They’ll spend a maximum of 18 weeks in the study, visiting the medical school once a week. As the subjects gain weight, researchers will test their fat cells, hormones and blood to find clues.

Klein said any study applicants who are found to be obese because of eating disorders will be referred for other help, rather than accepted for the study.

Also, people considered morbidly obese, more than 100 pounds overweight or a BMI of 40 or higher won’t be in the study, he said. “That’s another issue to itself,” Klein said.

One expert in weight management voiced cautious optimism about the study.

“There’s a benefit to knowing why some people have (metabolic problems) and others don’t; that’s a worthwhile question,” said Ginger Meyer, a registered dietitian with the Missouri Dietetic Association and the Weight Treatment Center with the Jefferson City Medical Group. The caution: “It’s important to not be put at risk in terms of the weight gain,” she said.

Mary Uhrich, study manager, said, “Participants are carefully watched by the medical team. They have a brief medical exam weekly throughout the intervention period.”

Also, the study is scrutinized for safety before it starts. “It has to be approved by an internal governing body at Washington University,” Uhrich said. “They carefully look over everything about the study, mainly taking the participants’ safety into careful consideration.”

If a participant shows he or she is developing a metabolic illness because of the study, Klein said, they’ll be pulled out and put in a weight loss program.